Turbine system having more failure resistant rotors and repair welding of low alloy ferrous turbine components by controlled weld build-up

ABSTRACT

System for repairing worn surfaces of steam turbine components and especially high pressure turbine rotors, are disclosed. These systems include depositing a first layer of weld metal on a worn surface of the component, whereby a heat-affected zone is created. A second layer of weld metal is then deposited over the first layer using a greater amount of heat to temper at least a portion of the heat-affected zone produced by the first layer. The preferred embodiments include the use of gas tungsten arc welding for providing fine-grain size and more creep resistance, especially in the weld and heat-affected zone. The resulting build-up can be machined, for example into a blade fastening to produce a component having properties equal to or better than the base-metal alloy. The invention also provides a longer lasting turbine system, including rotors which have serrated steeples that are more resistant to failure.

This is a division of Ser. No. 190,324, filed 5/5/88.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to co-pending application Ser. No. 168,097,filed Mar. 14, 1988, in the names of E. Clark and D. R. Amos, entitled"Repair Welding Low Alloy Turbine Components", which is assigned to theassignee of this application and which is hereby incorporated byreference.

This application is also related to co-pending application Ser. No.092,851, filed Aug. 24, 1987, in the names of R. T. Ward and J. M.Butler, entitled "Repair of High-Pressure Turbine Rotors By RingWelding", which is assigned to the assignee of this application andwhich is herein incorporated by reference.

This application is also related to application, Ser. No. 763,744, filedAug. 8, 1985, in the names of R. E. Clark and D. R. Amos, entitled"Method for Repairing A Steam Turbine Or Generator Rotor", now U.S. Pat.No. 4,633,544, issued Jan. 6, 1987, which is assigned to the assignee ofthis application and which is herein incorporated by reference.

This application is also related to co-pending application Ser. No.727,175, filed Apr. 25, 1985, in the names of R. E. Clark, D. R. Amos,and L. M. Friedman, entitled "High Strength, High Toughness Welding forSteam Turbine Rotor Repair", which is assigned to the assignee of thisapplication and which is hereby incorporated by reference.

1. Field of the Invention

This invention relates to repair procedures for worn or damaged surfacesof turbine components, and in particular, to welding techniques forbuilding up these worn surfaces with sound metal deposits.

2. Background of the Invention

Steam turbine components made of Cr-Mo-V alloys, such as rotors anddiscs, provide optimum high-temperature fatigue and creep properties,but are considered difficult to weld. However, since the down timeassociated with replacement of these often worn, eroded, or crackedcomponents can cost electric utilities hundreds of thousands of dollarsper day, many procedures have been attempted to repair them.

One such repair procedure consists of welding an individual piece offorged steel to a worn rotor or disc. However, when this type of repairis made on a single rotor blade groove fastening, herein referred to asa "steeple", welder accessibility is very limited. Accordingly, a weldrepair conducted with very limited accessibility can result inunacceptable, non-destructive examination quality due to the formationof porosity cracks and slag inclusions.

It is also known to make rotor repairs by submerged arc welding after alow volume welded seam is made between a turbine component and a forgedreplacement section. See Kuhnen, U.S. Pat. Nos. 4,213,025 and 4,219,717,which are herein incorporated by reference. In such a procedure, a ringforging is welded to a worn disc or rotor or a completely new rotorforging is welded to replace the entire end of the rotor. See Clark etal. U.S. Pat. No. 4,633,554, disclosing a narrow gap weld root passfollowed by a gas metal arc build-up for this purpose. The lower tensileand fatigue properties obtained by employing this process, however, aregenerally insufficient for use in high stress rotor steeple areas.

Submerged arc welding alone has also been used for build-up repairs ofrotor areas involving a wide or deep groove, where a cracked defect isnot oriented longitudinally along the radius of the rotor. The mainadvantage of building up with submerged arc welding is that thisprocedure has a very high deposition rate, typically about 15 pounds ofweld metal per hour. The higher deposition rate is important since manyof the service rotor weld repairs are made during turbine outages, thus,time is extremely important. However, this process requires a pre-heat,produces a relatively large grain size with inferior metallurgicalproperties. Typically, these submerged arc weldments on low pressurerotors have a yield strength of about 85 to 100 Ksi (586 to 689 MPa) anda room temperature Charpy toughness of about 100 to 120 ft-lbs (136 to163 J). It is also understood that submerged arc weldments are oftenrejected due to poor ultrasonic quality, which often reveals slaginclusions and porosity in the weld metal. Moreover, seriouscreep-rupture and notch-sensitivity problems have been encountered withhigh-pressure Cr-Mo-V rotor repair welds manufactured from submerged arcweldments. Thus, the submerged arc process is generally unacceptable forweld repairs of Cr-Mo-V rotor steeples having small, high-stressconcentration radii.

Gas metal arc procedures have also been employed for repairing rotorsand discs. This welding procedure deposits about 8 lbs of weld metal perhour, typically having slightly better properties than weldmentsproduced by the submerged arc process. For Cr-Mo-V rotor repair welding,the gas metal arc weldments of steel turbine components generally have ayield strength of about 85 to 100 ksi (586 to 689 MPa), and a roomtemperature Charpy toughness of about 110 to 130 ft-lbs (150 to 177 J).The gas metal arc welding process associated with welding these alloys,however, is often associated with arc-blow (magnetic) processlimitations which can limit the use of this process.

Recently, emphasis has been placed on the use Of gas tungsten arcwelding processes (GTAW) for making repairs on Ni-Mo-V and Ni-Cr-Mo-Vlow pressure rotor components. See R. E. Clark, et al. "Experiences withWeld Repair of Low Pressure Steam Turbine Rotors", 47th American PowerConference, Apr. 22-24, 1985, Chicago, Ill., printed by WestinghouseElectric Corporation, Power Generation, Orlando, Fla., hereinincorporated by reference. Gas tungsten arc welding has been employedfor repairing individual rotor attachment grooves, cosmetic, or shallowgroove repairs to correct minor surface defects. It has also been usedto allow multiple build-ups of plate attachment groove locations, i.e.,for a 360° application, and cladding to restore worn-away material. Gastungsten arc welding offers relatively high ultrasonic quality, requiresno pre-heat, and produces weldments having tensile and impact propertieswhich exceed rotor material specification requirements. Low allow steelweldments produced by this process typically have a yield strength ofabout 90 to 115 ksi (621 to 793 MPa), and a room temperature Charpytoughness of about 160 to 210 ft-lbs (218 to 286 J). In addition, thiswelding procedure produces the finest microstructural grain size of anyof the above-mentioned processes.

The selection of a weld method depends on factors such as distortion,non-destructive testing acceptance limits, and mechanical propertyresponse to the postweld heat treatment. Each area of a turbine rotor isunique, and experiences a different service duty. The absence of weldand heat affected zone cracking as well as the minimization of defects,can only be accomplished by carefully controlling a number of weldingvariables. For the gas tungsten arc welding process, some of thesevariables include amperage, alloy selection, joint geometries and travelrate. The parameters selected should be accommodating to automaticwelding processes to obtain a uniform quality which is reproducible fromweld to weld. These parameters must also produce superior weldingcharacteristics such as freedom from porosity, cracking, and slagentrapment, while being accommodating to all possible repairs on rotorsand discs. Finally, the alloy and welding parameters selected mustproduce a weld comparable to the properties of the base metal.

Accordingly, a need exists for a welding procedure that maximizes themetallurgical properties of the repaired area of turbine components.There is also a need for a welding procedure that minimize the heataffected zone and eliminates weld-related cracking.

SUMMARY OF THE INVENTION

Improved turbine systems including more failure resistant rotors andnovel methods for repairing worn surfaces of steam turbines, especiallyhigh pressure turbine rotors are disclosed. The methods include weldingprocedures and heat treatments that minimize weld stresses and cracking.The procedures employed substantially reduce the risk of failure inferrous Cr-Mo-V base metals of high-pressure, high temperature rotorsand discs commonly found in steam turbines. This invention presents animprovement over welding forged fastenings to rotors, since welderaccessibility and weldment integrity are improved. These features areparticularly important with respect to high pressure, (HP), turbinecomponents, such as rotors, which have been known to operate atpressures over 2400 psi and temperatures over 1000° F.

The invention includes depositing a first layer of weld metal on a wornsurface of a turbine component and then depositing a second layer ofweld metal over the first layer, using an higher applicationtemperature, for tempering at least a portion of the "heat-affectedzone" (HAZ) created in the base metal by the depositing of the firstlayer. As used herein, the term "heat affected zone" refers to the areaof base metal immediately adjacent to the fusion zone of the weldment.

Accordingly, improved welding methods are disclosed for overcoming theoccurrence of metallurgical structural problems within the heat-affectedzone. The additional heat generated by the deposition of the secondlayer of weld metal produces an immediate heat treatment of theheat-affected zone, whereby coarse grains of the base metal arerecrystallized and tempered. It is understood that when these coursegrains are reformulated into a finer grain structure, stress-reliefcracking in the vicinity of the weld repair can be minimized.

The methods employed by this invention also avoid the over-tempering, orsoftening, of the base metal created by the heat of welding the firstlayer of weld metal. This loss in strength occurs, to a greater extent,when a stress transverse to the weld is applied, for example, high andlow cycle fatigue, tensile, or creep-to-rupture. The proper control ofthe initial layers of weldment can significantly reduce the failure inthe heat-affected zone and prevent the loss of strength in this zonebelow the levels of the unaffected base metal.

Further improvements disclosed by this invention include the use of beadsequencing for minimizing heat input into the base metal. Run-off tabsare also taught for minimizing weld defects created by starting andstopping the arc. In addition, a weld trail-shield is disclosed forminimizing carbon losses in the base metal which could result in lowertensile properties. Finally, parameters such as preheat-interpasstemperatures, shield gas-type and flow rates, current, tungsten size andweld speed are also disclosed for achieving a higher quality weld.Procedures for single "steeple" repairs and for 360° rotor repairs arealso separately disclosed.

It is, therefore, an object of this invention to provide repair weldingprocedures compatible with high pressure, chromium-containing rotors andother components currently in service.

It is another object of this invention to provide welding procedures,alloys, and heat treatments which provide improve notch sensitivity andincreased creep ductility to repaired or new turbine components.

It is still another object of this invention to provide a repairedturbine rotor for use in high pressure service which is relatively freeof weld porosity, lack of fusion and cracking resulting from the weldingprocess.

With these and other objects in view, which will become apparent to oneskilled in the art as the description proceeds, this invention residesin the novel construction, combination, arrangement of parts and methodssubstantially as hereinafter described and more particularly defined bythe attached claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate a complete embodiment of theinvention according to the best mode so far known for the practicalapplication of the principles thereof, and in which:

FIG. 1: is a cross-sectional view of a control stage rotor wherein theold steeples have been machined off;

FIG. 2: is the cross-sectional view of the control stage rotor of FIG.1, illustrating a weld build up disposed on the machined surface;

FIG. 3: is the partial, cross-sectional view of the control stage rotorof FIG. 2, illustrating machined and repaired steeples;

FIG. 4: is a partial prospective of a single steeple repair technique,illustrating the use of run-off tabs and bead sequencing; and

FIG. 5: is a partial perspective, illustrating a 360° repair of a rotorwhere the steeples have been machined and the resulting 360° weldbuild-up is shown.

DETAILED DESCRIPTION OF THE INVENTION

The novel methods of repairing worn surfaces of ferrous turbinecomponents of this invention include providing a ferrous turbinecomponent 20, 40, or 50 having Cr, Mo and V alloying ingredients. Theseturbine components 20, 40, or 50 include worn surfaces, however it isexpected that new components can be manufactured using the methodsdisclosed herein. The repairing procedure includes depositing a firstlayer of weld metal the worn surface of the component thereby producinga heat-affected zone in that component. The procedure next deposits asecond layer of weld metal on top of the first layer. This second layeris deposited with a greater amount of heat than the depositing of thefirst layer for tempering at least a portion of the heat affected zoneproduced by the first depositing step. As used herein, the term"tempering" refers to the process wherein the heat-affected zone of thebase metal is reheated and then cooled to relieve internal stress andreduce its hardness.

By carefully controlling the weld parameters for the first to fivelayers of weld build-up, problems associated with the heat-affected zoneand resulting coarse grain structure of the base metal can be overcome.More specifically, using a gas tungsten arc weld procedure, the firstlayer is deposited using about 40 to 60 amperes, more preferably about60 to 140 amperes, and most preferably, about 80 to 120 amperes ofdirect current. The initial layer is welded at a relatively low amperageto create as small a heat-affected zone as possible. Next, the secondlayer of weld metal is deposited using about 50 to 200 amperes, morepreferably 75 to 175 amperes, and most preferably about 100 to 150amperes of direct current. Alternatively, alternating current could beemployed less advantageously.

The higher amperage associated with the application of the second layerof weldment has the effect of "heat treating" or tempering therelatively brittle heat-affected zone in the turbine component.Following this second layer, the third and fourth layers preferably areapplied using the same amount of current as used for applying the firstlayer. The fifth and subsequent layers can be applied using a highercurrent, since it will have less of an effect on the base metal.

It must be understood that the above current preferences are ideallysuited for a gas tungsten arc welding procedure using a wire size ofabout 0.045 inches, a 50% argon-50% helium gas mixture, and a 1/8 or3/32 tungsten size. It is expected that the tempering effect caused bythe depositing of the second layer could be accomplished by varyingthese and other parameters to obtain the same effect. For example, if asmaller wire size for the deposited alloy were used for depositing thesecond layer of weld metal, the amperage could remain about at the samesetting as the setting used for depositing the first layer. This, andother techniques, are well within the knowledge of those in the weldingart, and are mere variations of the principal teachings of thisinvention.

The ferrous alloys preferably used in this invention to repair theCv-Mo-V components of steam turbines are selected to include one or moreof the following elements: Cr, Mo, Si, C, Mn, V, Nb, and N. In a morepreferred embodiment, the ferrous alloy applied to the worn component isselected to comprise about 7.0 to 11.0 weight percent Cr and about 0.1to 3.0 weight percent Mo. One specified preferred ferrous alloycomposition range consists essentially of about 0.04 to 0.22 weightpercent C, 0.15 to 1.0 weight percent Mn, 0.15 to 1.0 weight percent Si,0.0 to 0.02 weight percent P, 0.0 to 0.016 weight percent S, 0.0 to 0.8weight percent Ni, 4.00 to 19.0 weight percent Cr, 0.43 to 2.1 weightpercent Mo, 0.09 to 0.5 weight percent V, 0.03 to 0.20 weight percentNb, 0.0 to 0.08 weight percent Al, 0.0 to 0.20 weight percent Cu, 0.005to 0.06 weight percent N and the balance being Fe. However, the mostpreferred a ferrous alloy consists essentially of 0.08 to 0.11 weightpercent C, 0.30 to 0.50 weight percent Mn, 0.30 to 0.50 weight percentSi, 0.00 to 0.10 weight percent P, 0.00 to 0.008 weight percent S, 0.00to 0.40 weight percent Ni, 8.00 to 9.50 weight percent Cr, 0.85 to 1.05weight percent Mo, 0.18 to 0.25 weight percent V, 0.06 to 0.10 weightpercent Nb, 0.00 to 0.04 weight percent Al, 0.00 to 0.10 weight percentCu, 0.01 to 0.03 weight percent N and the balance being Fe.

In further accordance to these novel methods, the depositing step canconsist of welding the ferrous alloy to the worn surface of the turbinecomponent. This welding step preferably is accomplished by any one ofgas tungsten arc welding, plasma-arc welding, electron beam welding,laser-beam welding and gas metal arc welding. It is expected that otherwelding procedures may be used to apply the novel alloys of thisinvention, however, it is important that the welding procedure employedminimize the heat-affected-zone in the base metal so as to avoidunnecessary defects.

The most preferred procedure employed thus far comprises gas tungstenarc welding (GTAW) the preferred ferrous alloys to a machined or groundturbine component. GTAW is preferred because its multiple beaddepositions exhibit exceptionally fine-grain size in the weld andheat-affected zone. This fine-grain size translates into exceptional lowand high cycle fatigue, tensile, impact and creep-to-rupture properties.

In accordance with the preferred gas tungsten arc welding procedures ofthis invention, the steam turbine component 20, 40, or 50 is preheatedto at least about 177° C. prior to the welding step. Side plates may beemployed for "360°" welding applications on discs and rotors. As usedherein, a "360°" repair refers to a procedure wherein weld metal isdeposited continuously about the circumference of a turbine component,such as a rotor or disc, until a sufficient height is reached whereuponthe individual steeples of the rotor are machined, or the disc ismachined down to service tolerances. The side plates preferably aremanufactured from Cr-Mo-V rotor steel or copper, and can be water cooledto further reduce welding side effects.

For a 360° steeple weld build up as described in FIG. 5, a preferredprocedure for high pressure rotor repairs, any steeples 44 present onthe rotor are machined off to the bottom of the blade grooves to formsolid ring of material. Next, a 360° weld build-up 54 is made byrotating the rotor under the weld torch, instead of moving the torchacross the width of the rotor disc, aS is done for individual bladefastening repair welds. For the multiple-type of repair weld, use may bemade of the above-mentioned water-cooled side plates which are disposedalong at least a longitudinal edge of the worn surface to contain theweld volume and minimize carbon loss in the weld deposit. The weldingoperation can alternatively deposit the preferred ferrous alloys of thisinvention against the water cooled side plates, if desired. When asingle steeple repair is made on a rotor component, the worn steeple ispreferably removed entirely from the rest of the rotor. Because theentire steeple is removed, the exceptional metallurgical propertiesassociated with the fine grain structure produced by the GTAW processare present throughout the finished blade fastening machined from theweld build-up. Next, a first run-off tab is disposed along at least afirst longitudinal edge of the rotor for at least providing a startingsurface for the welding step. A second run off tab can be disposed on asecond longitudinal edge of the rotor transversely opposed from thefirst longitudinal edge, for at least providing a surface for stoppingthe welding step. Since the attachment cites for the run-off tabs 46 cansometimes be the point of defects, a cladding procedure is used to jointhese run-off tabs 46 to the rotor 40. Preferably this claddingcomprises a buttering layer, containing chrome, and is disposed in atleast two overlapping weldments.

During a preferred (GTAW) welding repair of a single steeple repair, afirst bead is welded transversely across the rotor on the machined orotherwise prepared surface. Next, a second bead is welded transverselyacross the rotor on the prepared surface, and spaced apart from thefirst bead. The third and fourth weld beads are similarly disposed andspaced apart, if room is available. Using this intermittent weldingprocedure permits the immediate area of base metal underneath the weldto slowly cool prior to the next adjoining welding application.Accordingly, the brittleness associated with a weld-created, heataffected zone is minimized.

The turbine rotor 20 of FIG. 1 preferably is selected from a turbinealready in service, although it is expected that new rotors withoutserrations can be used as the initial turbine component for thefollowing welding procedures.

Generally the steam turbine rotors, discs and blades of this inventionare manufactured from low alloY steel, commonly containing less than 6%alloying elements. Of particular importance to these applications is theCr-Mo-V alloy, A 470, Class 8, and its modified versions. One mostpreferred composition includes 0.27-0.34% by weight C, 0.70-1.0% byweight Mn, 0.012 % by weight P and S (max), 0.20-0.35% by weight Si,0.50% by weight Ni (max), 1.05-1.35% by weight Cr, 1.00-1.30% by weightMo 0.21-0.29% by weight V, 0.15% be weight Cu (max), 0.010% by weight Al(max); 0.0015% by weight Sb(max), 0.015% by weight Sn (max), and 0.020%by weight As (max). Other forging alloys which can be used for makingsteam turbine components for high pressure service may also be repairedthrough the processes of this invention, such as those containingvarying amounts of Ni, Co, Cr and other alloying ingredients.

When a used turbine component, such as rotor 20, 40, and 50 areemployed, the highly stressed, individual steeples 44 are preferablymechanically removed. As used herein, "mechanically removing" refers toany of the known procedures for removing metal, including but notlimited to, grinding, machining, electric arc gouging, and other methodsknown to those in the metallurgy arts. As in the case of FIG. 4, theentire worn or damaged steeple should be removed since it is importantto reduce the possibility of creating any weak heat-affected zones inthe high stress areas of these components by subsequent weldingoperations.

As generally described in FIGS. 2, 4 and 5, the preferred ferrous alloycompositions of this invention can be deposited by welding them to theworn or damaged surface of the turbine component 20, 40 or 50 . Thiswelding step can be accomplished by any one of the known weldingtechniques, but preferably any one of gas tungsten arc welding,plasma-arc welding, electron beam welding, laser-beam welding, and gasmetal arc welding. A preferred preheat of at least about 100° C. toabout 300° C., more preferably about 177° C. to about 204° C., can beemployed for reducing stresses in the turbine component 20, 40, or 50prior to the welding step.

In preparation for the preferred gas tungsten arc welding procedure ofthis invention, the surfaces to be welded are preferably conditioned tobright metal. More preferably, the base metal surfaces are cleaned for adistance of about 2 inches from the weld area with denatured alcohol,acetone, methyl chloroform, or solvent cleaner. It is further noted thatif methyl chloroform is applied, it should be followed with an alcohol,acetone or solvent wash. It is also advised that the base metal surfaceto be welded be inspected using non-destructive testing procedures, andthat at least one sixteenth inch of additional metal be removed beyondthe deepest crack or fatigued area found.

In accordance with the preferred GTAW procedure of this invention, thefollowing welding parameters have been deemed useful:

                  TABLE II                                                        ______________________________________                                                                          Remainder                                   Parameter   Layer 1,3,4                                                                              Layer 2    of Weld                                     ______________________________________                                        Nature of Current                                                                         Pulsed 60% Pulsed 60% Straight                                                                      (no pulse)                                  Amperage-DCSP                                                                             85-120     100-150    280                                         Voltage     8.5-9.0    9.0-10     11-13                                       Surface Speed                                                                             4.0        3.0        4.5-8.0                                     (Linear), Inch                                                                Frequency of Pulse                                                                        3 cycles/sec                                                                             3 cycles/sec                                                                             None                                        Type of Travel                                                                            OSC        OSC        Straight                                    Osc. Amplitude                                                                            .22        .22         0                                          Wire Feed Speed,                                                                          5-25        5-20      50                                          inches per min.                                                               Tungsten Size,                                                                            1/8 or 3/32                                                                              1/8 or 3/32                                                                              1/8                                         2% Thorium, inch                                                              dia                                                                           Tungsten Stickout,                                                                        3/8-3/4    3/8-3/4    3/8-3/4                                     inch                                                                          Wire Size,  .045       .045       .045                                        inch dia                                                                      Primary Shield,                                                                           50% + 50%  50% + 50%  50% + 50%                                   Argon-Helium                                                                  Trail Shield,                                                                             100%       100%       100%                                        Argon                                                                         Bead Overlap                                                                               50%        50%        50%                                        ______________________________________                                    

As described in FIG. 4, the bead sequence for the welding applicationshould provide spaced bead sequencing, i.e., depositing a first bead 1transversely across the preferred rotor 40 on the machined surface andthen welding a second bead 2 on the prepared surface, transverselyacross the rotor 40 and spaced apart from the first bead. By working theweld metal from the outside to the inside of the welded areas for beads1-10, etc, less of a heat affected zone (HAZ) is created by the weldingprocedure. Peening is not advised, and the welding is preferablyaccomplished by an automated GTAW machine in the flat, +/-15° position.Weld stops for this machine shall be made using current tapering to avalue of 15 amps or less before breaking the arc. In addition, run offtabs 46, such those shown in FIG. 4, should be used for starting andstopping the weldment, since these locations may develop metallurgicaldefects. It is also advisable that the base metal be demagnetized priorto welding to minimize arc blow.

During the welding procedure, the interpass temperature of the basemetal preferably should be below 300° C., preferably below 250° C., andmost preferably below 204° C. Immediately after welding, the weldedturbine component 20, 40, or 50 and its weldment 12, 42, or 54 should bemaintained at a temperature of about 149° C. to about 260° C.,preferably about 176° C. to about 204° C. After this post-heatmaintenance schedule, the welded turbine component 20, 40, or 50 can begiven a post-weld heat treatment above 500° C., preferably above 600°C., and more preferably about 663° C. The post-weld heat treatmenttemperature should be selected to minimize weld stresses, providesufficient "tempering back" of the weld and heat affected zone hardness,and if necessary, prevent "over tempering" of the unaffected base metalto obtain the required weld strength. The preferred rotor repairtechniques of this invention generally include a post-weld heattreatment locally at the repair weld area. This local stress reliefconsists of heating the entire repair area and also axially along therotor to meet any preselected axial and radial temperature gradient.

Following the post-weld heat treatment of the welded area, the turbinecomponents 20, 40 and 50 repaired by the above-mentioned procedures areblast cleaned and provided with a nondestructive examination, forexample, magnetic particle, dye penetrant or ultrasonic testing. Inaddition, mechanical testing is conducted by determining the hardness ofthe weld deposit and by tensile testing metal coupons that werefabricated during the same welding operation. The turbine component isthen ready for final dimensional checks and a finishing machiningoperation, i.e. to produce individual serrations 14.

From the foregoing, it can be realized that this invention providesimproved procedures for repairing ferrous steam turbine components. Thewelding methods, alloys, and heat treatment schedules provide a repairedsurface exhibiting improved high temperature properties which mayinclude, for example, better creep and fatigue properties than the basemetal of such components. Although various embodiments have beenillustrated, this was for the purpose of describing, and not limitingthe invention. Various modifications, which will become apparent to oneskilled in the art, are within the scope of this invention described inthe attached claims.

I claim:
 1. In a steam turbine system of the type having a ferrous steamturbine component, said component having a worn surface thereon, theimprovement comprising:(a) a first layer of weld metal deposited on saidworn surface, said deposition of said first layer producing aheat-affected zone in said component; and (b) a second layer of weldmetal deposited to said first layer, said deposition of said secondlayer tempering at least a portion of said heat-affected zone in saidcomponent, wherein said turbine component comprises about 1.05% to 1.35%by weight Cr, about 1.00% to about 1.30% by weight Mo, and about 0.21%to 0.29% by weight V.
 2. The system of claim 1 wherein said turbinecomponent comprises a rotor.
 3. The system of claim 2 wherein said firstlayer of weld metal comprises a plurality of weld beads disposedtransversely across said rotor.
 4. The system of claim 3 wherein saidsecond layer of weld metal comprises a plurality of weld beads disposedtransversely across said rotor.
 5. The system of claim 4 furthercomprising steeples machined from a weld build up comprising at leastsaid first and second layers of weld metal.
 6. The system of claim 1wherein said steeple has a higher creep resistance than that of theferrous steam turbine component.